Cathode spot excitation



March 29, 1955 1 E, WHITE 2,705,293

CATHODE SPOT EXCITATION Filed Aug. 28. 1950.

lpg vai-f" AM/www i f Y i am# 1 Ir'fll'mlr'w @www JDhnBM/hite UnitedStates Patent O CATHODE SPOT EXCITATION John E. White, Silver Spring,Md., assigiior to the United States of America as represented by theSecretary of the Army Application August 28, 1950, Serial No. 181,907

15 Claims. (Cl. 313-193) (Granted under Title 35, U. S. Code (1952),sec. 266) The invention described in the specitication and claims may bemanufactured and used by or for the Government for governmental purposeswithout the payment to me of any royalty thereon.

This invention relates to gaseous discharge tubes and particularly tosuch a tube capable of handling very large currents.

In the electronics art, gaseous discharge tubes are commonly used asrelay devices, wherein the application of a small voltage to the grid ofa tube, or a small additional voltage in excess of a critical voltage tothe anode of a tube, causes a large amount of current to flow in thetube and its associated circuits. In any case, the current, of course,consists of electrons passing from the cathode to the anode. Current,therefore, is limited by a number of factors, two of the most importantbeing:

iirst, theability of a cathode of convenient size to emit` the requisitequantity of electrons; and, second, the ability of the tube and itselements to dissipate the heat caused by the passage ofthe current.

My invention consists of a novel type of cathode, as i hereinafterdescribed. I make use of a small directly heated cathode which formspart of a larger cathode. The larger cathode does not require heating asis usually understood, but releases a large quantity of electrons whensuch cathode is subjected to ion bombardment. The formation of a highlyemissive spot under such conditions is termed cathode spot excitation.`

My method of producing cathode emission by spot excitation differs fromthe usual thermionic cathode in several ways, and affords the followingadvantages:

a. My excited 'spot cathode makes it possible for gaseous dischargetubes to act as conductors for several tens of thousands of amperes, acondition not obtainable when the conventional thermionic cathodes areused.

b. It is not essential, after spot initiation that Imy excited spotcathode be heated by an external source, such as is required to maintainexcitation of a thermionic cathode.

c. My cathode is so designed, through the use of refractory conductivematerials and/or a means of keeping the spot in motion, that its life isnot greatly decreased by action of the cathode spot. l

d. My cathode is made of materials which are not only conductive andresistant to attack by the cathode spot, but which also facilitateelectron emission and spot formation.

e. My cathode is so shaped as to provide a local voltage drop whichaccelerates the emitted electrons and the received ions. This may bedone, for example, by constricting the cross section of the dischargenear the source of initiating electrons. I

f. The major advantage provided by my excited `spot cathode is that itaffords an extremely high upper limit of current for a given cathodesize. For example, the possible output from my cathode is of the orderof 2,000 to 4,000 amperes per square centimeter as compared to a maximumoutput of l to 5 amperes per square centimeter for the conventionaloxide-coated thermionic cathode.

g. Because only a portion of my cathode is heated by an external source,less external power is required for heating a cathode ot given poweroutput, as compared with the conventional thermionic cathode.

h. As the electron-emissitivity of my cathode is not dependent upon acoating on the surface of the material, but exists as well if thesurface layer is removed, it affords 2,705,293 Patented Mar. 29, 1955ICC longer working life than will the usual oxide-coated cathode.

i. Because of its more rugged construction, my cathode is less subjectto damage due to short circuits or other circuit faults.

The specific nature of the invention as well as other objections andadvantages thereof will clearly appear from a description of a preferredembodiment as shown in the accompanying drawing, in which the singlefigure is a schematic drawing of a circuit including a gaseous dischargetube illustrating the principle of my invention.

In the single ligure, there is represented a cathode, the major portionof which 1 is made of some such refractory material assintered tungstenand some such insulating material as thoria in proportions of the orderof 3 parts tungsten to l part thoria, using standard techniques, such,for example, as described in Therrnionic emission from sintered cathode,by H. l. Fan; `Iournal Applied Physics, July 1949, pages 682-690. Otherrefractory materials may also be used, such as thorium-molybdenum orthorium-rhenium. These parts are well mixed, and shaped under pressuresubstantially to the configuration shown. Within this major portionthere'is located a small thermionic portion 2 of the cathode, one end ofwhich is affixed and connected to the major portion 1, the other endbeing connected by means of a lead 2a to a source ofheating current 3,the circuit from this source of heating current being completed to themajor portion 1 of the cathode through an additional lead 2b.

Positioned approximately as shown in the figure is an anode 5, which isconnected to the operating load 6, and thence to a source of electricalpower 4, the circuit being completed back to the cathode through a leadto lead 2b. The gas used in the tube may be any suitable gas and in theusual pressure range commonly used for gas'lled thyratrons. A controlgrid 7 may be supplied for certain types of operation, as is well known.

Operation takes place when a suitable voltage is applied as shown at 4.When this voltage is below the l critical voltage, the tube isnon-conductive, the space between cathode 1, 2, and anode 5 forming anopen circuit. When, however, the voltage applied to the circuit at 4 isin excess of the critical voltage, a conductive path is formed between1, 2, and anode 5. Similarly, the` critical voltage can be reached witha lower applied voltage at 4 if current is caused to flow from source 3through the smaller portion of the cathode 2.

The action may be described as follows:

When cathode portion 2 is heated by the passage of electric current fromsource 3, electrons are emitted from cathode portion 2. These electronsare attracted by the positive bias on anode 5. Their passage through thegas which tills the cavity in portion l of the cathode, and the spacebetween this part and the anode 5, causes ionization of such gas. Bothpositive and negative ions and electrons are produced; the electrons areattracted to the anode, causing more collisions with gas molecules, andthus further ionization; the positive ions are attracted to the cathodeportion 1 by the negative charge resident thereon.

This bombardment of portion 1 by the ions causes an arc cathode spot tobe formed in region 1a; this spot emits a beam of electrons 8 fromcathode portion 1 to anode 5.

The formation of a cathode spot in region la is facilitated by twofeatures of this invention:

(l) The presence of insulating particles embedded in the cathodeproduces local high electric iield concentrations. This is caused by acharge built up on the insulating particles by bombardment of positiveions. Such high iields are known to cause the formation of a cathodespot.

(2) An arc formed by electrons from 2, passes through a constrictedregion in 1 in order to reach the anode. Such a constricted regionresults in a high voltage drop adjacent to 2, so that positive ionsstriking 1 will do so with considerable energy. This large energy favorsformation of a cathode spot.

The high arc drop resulting from the arc burning in constricted region1a causes the cathode spot to move rapidly out of the constrictedregion. This rapid motion of the spot prevents excessive local heating,and adds to the life of the cathode. It is also possible, if desired, toproduce additional cathode spot motion by suitable arrangement of amagnetic iield ina manne-r well known to the art. Such a method is oftenused to produce continued motion of a cathode'spot, as exemplified inthe Journal Applied Physics of March, 1950, page 193.

Although a 3-ele1nent tube, only, is shown in the ligure, my excitedspot cathode can be applied to any type of gaseous discharge tube, anditis not my intention to limit its application to the type shown.

I claim:

- 1. A gaseous discharge tube of the spot excited cathode typecomprising an envelope, a cathode and an anode in said envelope, saidcathode being in the form of a hollow member having an opening directedtoward said anode, said hollow member being formed of an intimatemixture of a metallic refractory conducting material and f an insulatingmaterial, a thermionic element electrically connected to the interiorsurface of said hollow member and to an external lead. on said envelope,and external leads on said envelope to said anode and cathoderespectively.

2. The invention recited in claim l, wherein said cathode insulatingmaterial is thoria. Y

3. The invention recited in claim l, wherein said refractory conductingmaterial is tungsten.

4. The invention recited in claim 3, wherein said hollow member is inthe form of a cylinder;

5. The invention recited in claim 4, and a control grid located betweensaid cathode and anode.

6. A gaseous discharge tube of the spot excited cathode type comprisingan envelope, a cathode and an anode in said envelope, said cathodeincluding a hollow, cylindrical member formed of a mixture of a metallicconducting, refractory material and of an insulating material of thegroup consisting of tungsten and thoria, thoriummolybdenum andthorium-rheninm, a thermionic element connected to said cylindricalmember at the inner surface thereof, and external leads connected tosaid cathode, said anode and said thermionic element through saidenvelope.

7. The gaseous discharge tube according to claim 6 further comprising atleast one control grid in said envelope intermediate said cathode andsaid anode, and ank external lead connected thereto through saidenvelope.

8. In a gaseous discharge tube of the spot excited cathode type, acathode consisting of a hollow, cylindrical y member formed of a mixtureof a metallic, refractory material and of an insulating material of thegroup consisting of tungsten and thoria, thorium-molybdenumandthorium-rhenium, and of a thermionic element connected to the in'nersurface of said hollow cylinder,` said hollow cylinder being open atboth ends. f

9, The apparatus` according to claim 8 wherein said metallic refractorymaterial is tungsten.

l0. The apparatus of claim 9 wherein said mixture is formed in ratio of3 parts of said refractory material to 1 part of said insulatingmaterial.

11. A gaseous discharge tube of the spot excited cathode type comprisingan envelope and at least a cathode and an anode in said envelope, saidcathode consisting only of a hollow cylindrical member formed of anintimate mixture of a refractory, conducting material of tungsten and ofan insulating material of thoria, said hollow cylindrical member beingopen in the direction of said anode, a thermionic element connected tosaid cathode at the inner surface thereof and leads connected to saidelement to pass a current therethrough.

12. A gaseous discharge tube of the spot excited cathode type comprisingan envelope and at least a cathode and an anode in said envelope, saidcathode including a hollow cylindrical member formed of an intimatemixture of a refractory, conducting material of tungsten and of aninsulating material of thoria, said hollow cylindrical member having itsaxis substantially directed toward the anode, a thermionic elementconnected to said cathode at the inner surface thereof and leadsconnected to said element to pass a current therethrough.

13. A spot excited cathode for a gaseous discharge tube comprising ahollow cylinder of sintered tungsten and thoria, a thermionic elementforming a minor portion of the said cylinder and means to pass a currentthrough said thermionic element.

14. A gaseous discharge tube of the spot excited cathode type comprisingan envelope and at least a cathode and an anode in said envelope, saidcathode comprising a hollow cylinder' formed of an intimate mixture oftungsten and thoria, a thermionie element forming a minor portion ofsaid cylinder, and leadsconnected to said element to pass an electriccurrent therethrough, said hollow cylinder having its axis substantiallydirected toward the said anode.

l5. A gaseous discharge tube of the spot excited cathode type comprisingan envelope and at least a cathode and an anode in said envelope, saidcathode comprising a hollow cylinder formed of an intimate mixture oftungsten and thoria in proportions of about 3 parts tungsten to one partthoria, a thermionic element connected to said cylinder and forming aminor portion of the said cathode, and means to pass a current throughsaid thermionic element, the said hollow cylinder having its axissubstantially directed toward the said anode.

References Cited in the file of this patent UNITED STATES PATENTS1,701,356 Bruckel et al. Feb. 5, 1929 1,814,759 McCullough July 14, 19311,993,811 Soundy Mar. 1,2, 1935 2,131,204 Waldschmidt Sept. 27, 19382,206,008 Spencer June 25, 1940 2,473,550 Spencer June 2l, 19492,501,089 Pomerantz Mar. 21, 1950

